Disorders of retinal vessel growth and function are responsible for vision loss in retinopathy of prematurity (ROP), a leading cause of vision impairment and blindness in childhood. ROP develops as a result of excessive growth of abnormal pre-retinal blood vessels, a compensatory mechanism that overcomes an earlier phase of hyperoxia-induced vaso-obliteration. Elucidation of the molecular bases of angiogenic cell function and behavior in physiological and pathological processes will have important therapeutic implications for the treatment of human retinal vascular diseases. The long term goal of our laboratory is to uncover the in vivo functions of the matricellular protein CCN1, also known as cysteine-rich protein 61, and the functional consequences of its expression, or lack thereof, in development and ischemic retinopathy. The CCN1 protein is an inducible immediate-early gene-encoded extracellular matrix (ECM) protein required for proper vascular development. In our preliminary studies, we have found that over-expression of CCN1 in the retina via either gene- or cell-based therapy, enhanced normal retinal vascularization and reduced pathological angiogenesis in the model of oxygen-induced retinopathy (OIR). In vitro data showed that CCN1 functions primarily through direct binding to specific integrins and ECM proteins, and/or indirectly through modulation of growth factor and Wnt protein expression and/or activity, thereby triggering signaling events that culminate in the regulation of cell adhesion, migration, proliferation, gene expression, differentiation, and survival. Our hypothesis is that CCN1 normalizes the biological mechanisms of retinal vessel formation during development and following OIR and overrides those leading to abnormal vessel formation. In Specific Aim 1, we will use mutant mice with inducible conditional inactivation of the CCN1 gene to determine how loss of CCN1 in endothelial cells (ECs) causes defective retinal vessel growth. We will identify interactions with Wnt- and Notch-derived signals known to influence functional specialization of ECs (tip and stalk cell phenotypes) and sprouting angiogenesis. We will further determine whether and how forced expression of CCN1 in ECs only, allows normal retinal vessel formation in OIR. In Specific Aim 2, we will define the molecular interactions of CCN1 with astrocytes, the primary proangiogenic cells responsible for retinal vessel formation and patterning during development, and we will determine the functional significance of CCN1 loss in astrocytes (and/or in ECs) in mutant mice on astrocyte activation state and behavior (e.g., migration, density and ensheathment) during development and in OIR. In Specific Aim 3, we will define the dynamics of the CCN1 promoter activity and identify the functional elements responsible for CCN1 gene modulation both in cultured retinal ECs subjected to hyperoxic stress and in retinas of OIR mice. This proposal will provide new insights into the molecular mechanisms of CCN1 activities in vivo and may foster future safer, less destructive, and more effective therapies to harness ischemia-induced neovascularization in ROP.